Atmos. Chem. Phys., 10, 7533–7544, 2010
www.atmos-chem-phys.net/10/7533/2010/
doi:10.5194/acp-10-7533-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Transport of North African dust from the Bodélé depression to the
Amazon Basin: a case study
Y. Ben-Ami1 , I. Koren1 , Y. Rudich1 , P. Artaxo2 , S. T. Martin3 , and M. O. Andreae4
1 Department
of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot, Israel
of Physics, University of São Paulo, Brazil
3 School of Engineering and Applied Sciences & Department of Earth and Planetary Sciences, Harvard University,
Cambridge, Massachusetts, USA
4 Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
2 Institute
Received: 26 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 12 February 2010
Revised: 23 July 2010 – Accepted: 26 July 2010 – Published: 16 August 2010
Abstract. Through long-range transport of dust, the NorthAfrican desert supplies essential minerals to the Amazon
rain forest. Since North African dust reaches South America mostly during the Northern Hemisphere winter, the dust
sources active during winter are the main contributors to the
forest. Given that the Bodélé depression area in southwestern
Chad is the main winter dust source, a close link is expected
between the Bodélé emission patterns and volumes and the
mineral supply flux to the Amazon.
Until now, the particular link between the Bodélé and
the Amazon forest was based on sparse satellite measurements and modeling studies. In this study, we combine a
detailed analysis of space-borne and ground data with reanalysis model data and surface measurements taken in the central Amazon during the Amazonian Aerosol Characterization
Experiment (AMAZE-08) in order to explore the validity and
the nature of the proposed link between the Bodélé depression and the Amazon forest.
This case study follows the dust events of 11–16 and 18–
27 February 2008, from the emission in the Bodélé over West
Africa (most likely with contribution from other dust sources
in the region) the crossing of the Atlantic Ocean, to the observed effects above the Amazon canopy about 10 days after
the emission. The dust was lifted by surface winds stronger
than 14 m s−1 , usually starting early in the morning. The
Correspondence to: Y. Ben-Ami
(yuval.ben-ami@weizmann.ac.il)
lofted dust, mixed with biomass burning aerosols over Nigeria, was transported over the Atlantic Ocean, and arrived over
the South American continent. The top of the aerosol layer
reached above 3 km, and the bottom merged with the boundary layer. The arrival of the dusty air parcel over the Amazon
forest increased the average concentration of aerosol crustal
elements by an order of magnitude.
1
Introduction
Mineral dust has been suggested to play an important role
in biogeochemical cycles (Falkowski et al., 1998; Garrison
et al., 2003; Jickells et al., 2005, Mahowald et al., 2009),
climatic processes (Twomey, 1974; Levin et al., 1996; Albrecht, 1989; Falkowski et al., 1998; Ramanathan et al.,
2001; Rosenfeld et al., 2001; Koren et al., 2005; Kaufman
et al., 2005c; Teller and Levin 2006; Jiang et al., 2006; Stith
et al., 2009), and human life (Prospero, 1999; Griffin and
Kellogg, 2004).
Despite major efforts, the spatial and temporal distributions of mineral dust remain uncertain. Therefore, the nature
and magnitude of dust feedbacks on climatic processes, biogeochemical cycles, and human life are not well constrained.
Important unanswered questions include: what are the exact
North African dust sources of crustal elements observed in
the Amazon Basin and what are their associated fluxes?
Published by Copernicus Publications on behalf of the European Geosciences Union.
7534
The Amazon rain forest has important roles in Earth’s
climate system. It acts as a source of primary and secondary biogenic aerosol particles and components (Artaxo
and Hansson, 1994; Quéré et al., 2009; Martin et al., 2010).
It also affects the global biogeochemical cycles, including
carbon cycling (Davidson and Artaxo, 2004). To sustain the
well-being of the forest and the fragile balance between the
rain forest and the atmosphere, the Amazon forest must receive a sufficient amount of nutrients. However, the intense
precipitation and ensuing floods wash most of the soluble
minerals from the rainforest soil, leaving behind a quite infertile soil, with limited nutrients available for plants growth
(Vitousek and Sanford, 1986). In-situ measurements (Talbot
et al., 1990) and image analysis studies (Swap et al., 1992)
have suggested that the deficiency of nutrients in the Amazon
soil can be replenished by deposition of mineral dust, mostly
from the Sahara.
Mineral dust aerosol were previously observed in the
Amazon region, and its appearance was attributed to arrival
of air parcels from North Africa (e.g.: Prospero et al., 1981;
Artaxo et al., 1990; Swap et al., 1992; Formenti et al., 2001).
Nevertheless, not all dust outbreaks reach the Amazon forest. Transport of the North African dust across the Atlantic
Ocean was studied since the early seventies. The active dust
sources, the trajectories across the Atlantic Ocean, as well as
the sink of North African dust, are subjected to the seasonal
cycle due to the shift in the location of the inter tropical convergence zone. During the late boreal winter and the spring,
North African dust, mixed with biomass-burning aerosols
from the Sahel region (Formenti et al., 2008; Ansmann et
al., 2009), follows southern trajectories heading to the northern part of South America (Kaufmann et al., 2005a; Huang
et al., 2010). The transport of dust over the Atlantic Ocean
(about 7 days) was intensively studied by ground based (e.g.:
Prospero et al., 1981, 1999) and airborne measurements (e.g.:
Prospero and Carlson, 1972; Carlson and Prospero, 1972;
Reid et al., 2002), satellite remote sensing (Karyampudi et
al., 1999; Torres et al., 2002; Kaufman et al., 2005a; Liu
et al., 2008a and b; Generoso et al., 2008; Ansmann et al.,
2009; Ben-Ami et al., 2009; Huang et al., 2010) and transport models (e.g.: Ginoux et al., 2004; Schepanski et al.,
2009; Engelstaedter et al., 2009).
The arrival of the dust to the Amazon forest is manifested
by an increase of crustal elements such as Al, Si, Fe, Ti, and
Mn, generally from the submicron fraction (e.g.: Prospero et
al., 1981; Artaxo et al., 1990; Swap et al., 1992; Formenti et
al., 2001; Martin et al., 2010). The peak of crustal elements
range from one to several days, and is typical to the months
of March and April, which is the wet season in the central
part of the Amazon Basin. Kaufmann et al. (2005a) estimated that 50±15 Tg y−1 of dust reaches the Amazon Basin.
Transport of biomass smoke from Africa occurs also during
the dry season (Martin et al., 2010). The above elements
serve as markers for the arrival of mineral dust and they are
not necessarily essential for plants growth. The bioavailabilAtmos. Chem. Phys., 10, 7533–7544, 2010
Y. Ben-Ami et al.: Transport of North African Dust
ity of elements contained in mineral dust to plants is beyond
the scope of the paper.
Although the transport route of Saharan dust was extensively studied, the exact origin of the dust, as well as the magnitude and the frequency of transport are not well known. An
important reason for these uncertainties is that previous studies focused either on the region of the Amazon forest and the
transport route over the Atlantic Ocean, or over North Africa
, and so the direct link, between the source and the sink locations was not studied in sufficient detail.
Koren et al. (2006) suggested that the Bodélé depression
(centered in Chad, 17◦ N 18◦ E), is the main source for the
dust transported to the Amazon Basin. Using a year-long
satellite data set, they studied the emission patterns of the
Bodélé, and suggested a dust transport route towards the
Amazon. However, their study did not include ground-based
measurements, and their conclusions were based on inductive reasoning, showing that the North African dust arrives in
the Amazon mostly during the (Northern Hemisphere) wintertime and that the Bodélé is the main winter source. Therefore, they concluded that the Bodélé contributes a significant
amount of nutrients to the Amazon Basin.
Other regional (Schepanski et al., 2007; Ginoux et al.,
2009) and global (e.g.: Prospero et al., 2002) remote sensing
studies also identified the Bodélé depression as one of the
most active dust sources. The Bodélé depression is characterized by a year-long activity and a dust emission peak during the Northern Hemisphere winter months, with estimated
emissions of (58 ± 8) × 106 tonnes (estimated for winterspring of 2003–2004), equivalent to emissions of more than
7 × 105 tonnes per emission day (Koren et al., 2006). The
uniqueness of the Bodélé as a prime dust source was attributed to three factors: A. Mineralogy – being part of the
former lake Mega-Chad, the Bodélé (∼ 133532 km2 ) is a rich
dust reservoir, including low density diatomite and eroded
diatomite sand (Bristow et al., 2009), composed of SiO2 and
small quantities of Al2 O3 and Fe (Chappell et al., 2008). The
importance of the Bodélé in fertilizing the Amazon Basin
with Fe and P was addressed by Bristow et al. (2010); B.
Meteorology – The Harmattan winds form the Low Level
Jet (Washington and Todd, 2005) and, C. Topography – the
persistent surface winds blowing southwestward are focused
toward the Bodélé depression by the structure of the valley
between the Tibesti (∼ 2600 m) and Ennedi (∼ 1000 m) massifs (Koren et al., 2006).
During the winter months, dust emitted from the Bodélé
depression is transported by the Harmattan winds southwestward over the Sahel toward the Gulf of Guinea into the Atlantic Ocean heading to the coast of South America (Kaufman et al., 2005a).
The extensive emitted dust mass, the tropical Atlantic
northwesterly winds regime, and the observed crustal elements found in dust in the Amazon Basin suggest that North
African dust reaches the Amazon forest. The exact North
African dust sources that actually the dust is originating
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Y. Ben-Ami et al.: Transport of North African Dust
7535
from, however, remains challenging. In contrast to previous
studies who suggested the arrival of North African dust to the
Amazon forest but showed no clear evidence for the exact
source of the dust, the objective of this study is to use ground
based measurements and remote sensing observations in order to show clear connection between dust source in North
Africa and the Amazon basin. Herein, we present a detailed
case study that integrates the data analysis approach to track
Bodélé’s dust to the Amazon region.
2
Methods
The objectives of AMAZE-08 were to understand the sources
and processes that regulate emissions, transformation and deposition of biogenic aerosol particles in the forest. Extensive ground based measurements were conducted between
7 February and 14 March 2008 during the wet season. The
AMAZE-08 site was 60 km NNW of Manaus and located
within a mostly pristine rainforest (Chen et al., 2009; Martin
et al., 2010).
We analyzed North African dust emissions and transport
along the dust route during this period, using surface and remote sensing measurements. The surface wind speed and
azimuth were estimated by probing the propagation of a dust
front between the morning overpass of the MODerate resolution Imaging Spectroradiometer (MODIS) instrument on
board Terra and the afternoon MODIS-Aqua satellites measurements, using the 1 km resolution blue band (440 nm) data
(Koren and Kaufman, 2004). Time of emission in Africa was
estimated by back propagating the dust front to the source,
using the derived wind speed. Dust passage was monitored at
the Ilorin AErosol RObotic NETwork (AERONET, Holben et
al., 1998) station located in Nigeria (8◦ 190 12 N, 4◦ 200 24 E),
along the dust pathway.
Dust mass was estimated at two locations, close to the
source and over the ocean:
A. Near dust sources, which are far from any others
aerosol sources, the observed Aerosol Optical Depth (AOD)
was attributed solely to dust. The dust AOD was transformed
to dust mass, Mdu , using the following relation:
Mdu = 2.7Aτdu (g)
(1)
where A is the plume area (in 1 km resolution) and τdu is the
mean dust AOD at the 550 nm band (at 10 km resolution) of
the MODIS instrument (based on the deep blue data (Hsu et
al., 2004)) located on Aqua. The factor 2.7 (±0.4) g m−2 is
a result of regression calculations between the AOD in the
visible (550 nm) and aerosol column concentration, based on
several in-situ measurements in the North Africa region. The
uncertainty in the mass calculations is ±30% (for total AOD
between 0.2 and 0.4, see Kaufmann et al., 2005a and the related references listed therein; Yu et al., 2009). Note that in
cases when total AOD > 0.4, the error may be larger.
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B. Over the Atlantic Ocean, it is likely that dust particles
are mixed with marine aerosol (sea salt, organic matter and
oxidation products of dimethyl sulfide (Yu et al., 2009)) and
biomass-burning aerosols from the Sahel region (Andreae et
al., 1994; Formenti et al., 2008; Ansmann et al., 2009). This
mixing increases the fine fraction, f , defined as the aerosol
fraction with diameter smaller than 1 µm (Kaufmann et al.,
2005b). Therefore, over the ocean, the daily dust mass, Mdu ,
was estimated from Eq. (1), where A is the area over the
ocean covered by dust (between 20◦ N–15◦ S, and 50◦ W–
15◦ E). τdu was extracted from the total AOD, τ , and from f ,
using Eq. (2) (Kaufmann et al., 2005a; Yu et al., 2009):
τdu =
τ (fa − f ) − τm (fa − fm )
fa − fd
(2)
τ and f were products of the 550 nm band of the MODIS
instrument (both in 1◦ resolution) located on the Terra and
Aqua satellites. The fine fraction, f , is assumed to be composed of 3 main types of aerosol, as described by Eq. (3)
(Kaufman et al., 2005a)
f = fa + fm + fd
(3)
were fa , fm , fd are the anthropogenic, maritime and dust
fine fractions (0.90, 0.45 and 0.37, respectively, with estimated error of 20%) as estimated over selected regions (Yu
et al., 2009). The AOD attributed to maritime aerosol, τma ,
was based on Eq. (4) (Smirnov et al., 2003; Kaufmann et al.,
2005a):
τma= 0.007W ± 0.02
(4)
where W is the magnitude of the wind speed at 1000 hPa,
obtained from the National Center for Environmental Prediction reanalysis (Kalnay et al., 1996).
The propagation of the dust/biomass-burning plumes was
further analyzed by studying the polarization signal of the
plumes, using the Cloud-Aerosol LIdar with Orthogonal
Polarization (CALIOP) instrument on board the CloudAerosol Lidar and Infrared Pathfinder Satellite Observation
(CALIPSO) satellite (Thomason et al., 2007). In order to
enhance the backscatter signal, the vertical (up to 8 km) and
the horizontal resolution of the profiles were reduced to 60 m
(2 signal points) and 5 km (15 signal points), respectively. In
contrast to other types of aerosols, dust particles are dominated by no spherical particles, and therefore their Volume
Depolarization Ratio (VDR, i.e., the ratio of the perpendicular to parallel components of the attenuated backscatter at
532 nm, including aerosol and molecular scattering) is expected to be relatively high, approximately between 0.1 to
0.4 (Murayama et al., 2001; Liu et al., 2008a, b, c).
The distinction between aerosols and clouds was based
on (a) the VDR and (b) the different backscatter patterns
(Ben Ami et al., 2009): we assumed that clouds have strong
backscatter signal, and either sporadic and large vertical dimension (cumulus clouds) or continuous horizontal shape
Atmos. Chem. Phys., 10, 7533–7544, 2010
7536
Y. Ben-Ami et al.: Transport of North African Dust
(stratiform clouds). Aerosol plumes, on the other hand, have
a continuous horizontal and vertical structure with wide horizontal dimension and weaker backscatter signal.
The propagation of the dust over the Atlantic Ocean was
also studied by analyzing the spatial distribution of the AOD
from MODIS along the center of the dust/biomass-burning
plumes (between 10◦ N–5◦ S, and 50◦ W–10◦ E).
At the AMAZE-08 field station (2◦ 350 2200 S,
60◦ 060 5500 W), aerosol particles having diameters smaller
than 2.0 µm as well as those in the size range from 2.0 to
10 µm were collected from above the rain forest canopy,
using Stacked Filter Units with Nuclepore filters. Elemental
analysis of collected particles was carried out by Particle
Induced X-ray Emission (PIXE) (Artaxo et al., 1987).
The transport of the dust, from the Bodélé depression towards the Amazon Basin, was estimated also by trajectories of the dust-containing air parcels via the HYbrid SingleParticle Lagrangian Integrated Trajectory (HYSPLIT) model
(Draxler and Rolph, 2003).
3
a.
b.
Sahara
1
Sahel
2
N
150 km
c. 18 February 2008
Dust plumes from
additional dust sources
Dust plumes from
the Bodélé
Results
The results are presented in four stages, from source to sink,
showing correlations that link each subsequent stage to the
previous one:
1. Analysis of the Bodélé depression emission pattern.
2. Analysis of aerosol properties over the Ilorin
AERONET station near the Gulf of Guinea.
3. Analysis of the transport of air masses over Africa and
over the Atlantic Ocean.
d. 19 February 2008
Mixed plume: Dust
plume from the
previous day, possibly
mixed with biomass
smoke and fresh dust
from dust source along
the route
More dust
emitted from
the Bodélé
4. Elemental analysis (by PIXE) of aerosols collected at
the AMAZE-08 field site in Central Amazonia.
5. Forward trajectories of air parcels originated from the
Bodélé depression.
3.1
Analysis of the Bodélé depression emission pattern
During the period of this study, between February 2008 and
mid March, the Bodélé depression was the most active dust
source in North Africa (Fig. 1). On February 2008, the activity in the Bodélé depression was characterized by two periods
of dust emission between 6–8 and 11–16 February, followed
by an extensive emission between 18–27 February. During
early and mid-March, the Bodélé was significantly less active. Additional dust activity with much smaller magnitude
was observed in other nearby sources. On a few occasions,
the activity in the Bodélé was accompanied by significant
emissions from other sources along the Moroccan coastline
(not shown). These plumes, for the most part, propagated
northwest, away from the South American coastline. Figure 1 illustrates the dominance of the Bodélé depression, in
comparison to other dust sources.
Atmos. Chem. Phys., 10, 7533–7544, 2010
Fig. 1. (a) The location of the Bodélé depression (marked with red);
(b) The Bodélé depression (composed of two main dust sources,
marked in red) on a clear day (5 March 2008); (c–d) daily images
of North Africa as seen by the MODIS instrument on board the
Aqua satellite (collection 005) for 18 and 19 February, for the region
between 37◦ N–5◦ S and 17◦ W–35◦ E. Blue regions marked area
with no satellite coverage. The gaps between two aerosol plumes
are equivalent to emission less periods (usually nighttime). The
images are taken from http://modis-250m.nascom.nasa.gov/.
Frequently, the dust originated from the Bodélé is emitted
from two sources: the main source area, northeast and within
the large ephemeral lake (point 1 in Fig. 1b) and a second
smaller source area ∼ 150 km south of the lake which is not
always active (point 2 in Fig. 1b).
The emission pattern analysis is based on five emission
days (16, 20, 24, 25 February and 22 March) that were selected based on MODIS coverage during the study period.
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Y. Ben-Ami et al.: Transport of North African Dust
7537
4
1
0.8
0.6
2
0.4
Angstrom Exponent
Aerosol Optical Depth
(500 nm)
3
1
0.2
18
0
29/2
16
27/2
14
25/2
12
23/2
10
21/2
8
19/2
6
17/2
4
15/2
2
13/2
0
Fig. 2a. Level-2 AERONET AOD at 500 nm and Angstrom exponent for 440–870 nm during the period between 12 February and
1 March 2008. Periods of high AOD and low Angstrom exponent
are marked with yellow rectangles.
The emission started during the early morning hours, approximately between 03:30 to 06:30 (local time), with surface wind speeds between 14.5 to 18 m s−1 and wind azimuth
between 225◦ and 250◦ . The emitted plumes, observed by
the Aqua satellite, covered an area between 12 × 103 and
100 × 103 km2 . The average AOD of the plumes varied between 2 and 3.9 (most likely underestimated), and the emitted mass per emission day (up to about 12:30) varied between
1 × 105 and 1 × 106 tonne. The emitted mass was equivalent
to an emission flux between 2.7–40 t s−1 (based on different
durations of emission).
The estimated time of emission, surface wind azimuth and
emission flux are in agreement with the analysis of Koren and
Kaufman (2004) and Koren et al. (2006). The estimated surface wind speed is slightly higher than measurements done
by Koren and Kaufman (2004) and Koren et al. (2006); however, it is in line with the suggested minimum threshold speed
for dust emission of 10–11 m s−1 (Koren and Kaufman 2004;
Todd et al., 2007).
A special case is the dust emission during 18 February,
which was characterized by early emission (starting approximately at 23:30) and a large emitted mass of ∼ 2 × 106 tonne
(up to noon local time). The extensive dust emission continued on 19 February. The emission of the Bodélé then persisted for seven more days until 27 February.
3.2
Analysis of aerosol properties over the Ilorin
AERONET station.
Focusing on two emission periods, 11–16 and 18–27 February, we followed the dust plumes along their routes westward toward the Gulf of Guinea. During 14–16 and
20–22 February, the Ilorin AERONET station (located at
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Fig. 2b. Back trajectories starting from the Ilorin AERONET station (white circle) during 14–16 and 20–22 February, starting at
1000 m (red) and 2000 m (blue) above ground level (starting at
12:00 UTC), calculated via HYSPLIT model. The location of the
Bodélé depression is marked with white star.
8◦ 190 N, 4◦ 200 E) measured a significantly increased AOD
(most likely underestimated), accompanied by a decrease of
the Angstrom exponent (Å, Eck et al., 1999). Small Å values
indicate the presence of large particles such as dust. Significant emissions, starting on 18 February, are reflected in
the AOD values (Fig. 2a). Back trajectory calculations from
Ilorin station, during 14–16 and 20–22 February, suggest that
the air masses passed over the Bodélé region (Fig. 2b) and
that the transport time was ∼ 2.5 days.
3.3
Analysis of the transport of air masses over Africa
and over the Atlantic Ocean
The propagation of the dust plumes over North Africa was
analyzed using CALIOP vertical profiles. Over the Atlantic
Ocean, we used CALIOP and MODIS measurements. Figures 3a and b show the propagation of the dust/biomassburning plumes during the study period, beginning near the
source, continuing over the Atlantic and finally toward the
Amazon Basin. It can be seen that the dust event is composed of two major plumes (as observed by MODIS, Fig. 1c
and d) with top heights near 1.2 and 2 km (for 19 February), both emitted from the Bodélé. As the dust propagated
southwest from the Bodélé, it mixed with biomass burning
smoke. From the mixing region over Nigeria, the dust and
the biomass smoke continued together, as shown in the VDR
analysis. The comparison between Fig. 3a and b with observations from a day without a dense aerosol plume (Fig. 4)
clearly emphasizes the presence of the dust and the biomassburning aerosols over the marine boundary layer during the
period of the study.
Atmos. Chem. Phys., 10, 7533–7544, 2010
7538
Y. Ben-Ami et al.: Transport of North African Dust
0.1
0.01
0.001
0.1
0.0001
0.01
0.001
0.0001
b.
a.
Regions where the signal
of the LIDAR diminished
due to dense clouds from
above
5 km
5 km
S. America
S. America
Feb 24
Feb 26
Feb 23
Feb 25
Feb 22
Africa
Feb 24
Feb 21
Feb 21
Feb 20
Feb 20
Africa
Feb 19
Feb 19
Equator
2.5
Lat. 0 1.10
Long. 9.45
Biomass burning and
Dust (see section 3.3.1
and Figure 6a).
10.20
12.50
Dust plume from
source #2 in the
Bodélé. (Marked on
Figure 1b)
21.30
5.25
Dust plume from
source #1 in the
Bodélé. (Marked on
Figure 1b)
Fig. 3a. CALIOP nighttime vertical attenuated backscatter profiles
(km−1 s−1 ) in the 532 nm band, plotted over the study area. The
vertical profiles are 5 km in height (February 2008). The location
of the Bodélé is marked by a yellow star. The lower enlargement
shows the interaction region between the biomass-burning smoke
and the dust plumes. The dust is recognized by the strong backscattering signal relative to the biomass-burning plume (the data was
verified with MODIS).
3.3.1
Equator
5
2 km
1.2 km
Height (k m)
Height (k m)
Feb 18
5
2.5
0
Lat. 4.60
Long. 16.85
10.33
13.10
Biomass burning
and dust
22.80
10.10
Dust
Fig. 3b. As Fig. 3a, but for daytime profiles and for 1064 nm wavelength.
Volume depolarization ratio analysis
VDR, retrieved from the vertical profiles of CALIOP, is a
function of CALIPSO wavelength and the size and shape of
the targets particles and air molecules. Therefore, the acquired VDR is in fact a result of different types of interactions.
The histogram of the area covered by the aerosol plumes
showed that most of the values fell between ∼ 0.1 to ∼ 0.4
(Fig. 5). These VDR values, which are typical for dust
(Liu et al., 2008a, b, c; Tesche et al., 2009), indicate that
the majority of the observed signal (during the period of
the study) was obtained from dust particles. The obtained
values are also in agreement with results of 0.31 ± 0.03
(Freudenthaler et al., 2009) attributed solely to dust particles. The relatively high values, namely more signal at the
perpendicular band of CALIOP, derived from the irregular
Atmos. Chem. Phys., 10, 7533–7544, 2010
Fig. 4. A CALIOP nighttime vertical attenuated backscatter profile
(km−1 sr−1 ) in the 532 nm band from 2 February 2008: An example of a CALIOP profile where one see the height of the marine
boundary layer (marked by the height of the marine stratocumulus
clouds) with almost no presence of dense aerosol above it. Aerosols
above the marine boundary layer are marked by an ellipse. The
CALIPSO track and the direction of the flight are shown on the upper part (red).
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Y. Ben-Ami et al.: Transport of North African Dust
250
0.1
0.25
7539
0.4
Occurrence frequency
200
150
100
50
0
0
M
A
B
D
0.1
0.2
C
0.4
0.6
0.8
Volume depolarization ratio
1
Fig. 5. Frequency of occurrence of VDR, obtained from the vertical area covered by the aerosol plumes for the area between 40◦ N
and 5◦ S and between 17◦ E and 46◦ W as seen in Fig. 3a (based
on CALIOP nighttime profiles between 19–24 February 2008). The
edges of the plumes are defined based on the VDR and different
backscatter patterns (see Sect. 2). Aerosols located in the marine boundary layer (for which the height of the marine boundary
layer was defined based on the location of marine stratocumulus
clouds, Ben-Ami et al., 2009) were excluded from the analysis, and
therefore the VDR obtained from sea salt is expected to be minor.
VDR regions dominated by air molecules (M), anthropogenic (A),
biomass-burning (B), dust (D) aerosol and clouds (C) are marked
in red. Note that the small peak of VDR < 0.1 was mostly from
data taken on 19 February (see Fig. 6b). The threshold for pure dust
(0.25) is marked by a green line.
shape of the dust and from multiple scattering. Note that in a
case of high AOD the signal of CALIPSO, obtained from the
lower part of the aerosol plume, may have a lower quality.
Water clouds located near or within the aerosol plume,
have higher VDR values (between 0.4 to 1) due to multiple scattering and were a minor contribution to the overall
sampled signal. Anthropogenic aerosols, such as biomass
smoke and molecules, which are much smaller compared to
the CALIOP wavelength, have low VDR values, between 0
to 0.1 (Liu et al., 2008 a, b, c).
Most of the signal is attributed to dust since most of
the VDR data is within 0.1–0.4. To emphasize the location of cleaner dust within the transported aerosol plume,
we showed regions where the VDR is larger than 0.25 (the
threshold is marked on Fig. 5) and smaller than 0.4. This
relatively high value indicates that the selected regions are
dominated by dust. Based on this assumption, several regions of relatively pure dust, as well as regions characterized
by biomass burning mixed with dust (with lower VDR values) are marked in Fig. 6a. Close to the source (19 February), the aerosol plume is composed of (a) biomass burning
mixed with dust in the southern parts of the aerosol plume
(VDR of 0.19), (b) relatively pure dust in the central part of
www.atmos-chem-phys.net/10/7533/2010/
the plume (VDR of 0.31) and apparently (c) anthropogenic
aerosol (VDR between 0 to ∼ 0.13), most likely transported
from North Libya and/or Europe over the Mediterranean
(Duncan et al., 2008), in the northern part (Fig. 6a). This assumption was supported by back trajectories from HYSPLIT
model, showing that the air parcel from the northern part
of Fig. 6a (for 19 February, between latitudes ∼ 22 N◦ and
∼30 N◦ ) passed over Europe and the Mediterranean, heading central North Africa (not shown).
The tri-modal distribution of these aerosols is also evident
in Fig. 6b. The VDR values of ∼ 0.3 are in line with previous
studies, and the VDR value of ∼ 0.19 is significantly larger
than that of < 0.1 expected for pure biomass smoke (Liu et
al., 2008a, b, c), indicating that the biomass smoke is mixed
with dust particles.
The mixing of both types of aerosol over the Atlantic
Ocean, as well as sedimentation of large dust particles along
the transport route, is also evident from the shift of the center
of the VDR distribution (Fig. 6b) from 0.32 over Africa to
0.15 over the western Atlantic.
Detailed analysis of the regions that are dominated by
dust (marked with red in Fig. 6a) shows that the VDR decreases over time during transport, possibly due to extensive sedimentation of large dust particles. An opposite trend
of increasing VDR (from 0.19 to 0.22) characterized the
regions that are dominated by biomass burning mixed with
dust (marked with green on Fig. 6a), indicating that more
dust is dissipating toward the air mass containing the biomass
burning. Further west (22–24 February), the VDR decreases
again, most likely due to aerosol wet removal and sedimentation.
3.3.2
Analysis with MODIS AOD
As the dust plume continued to move away from its source,
the spectral signal of the dust weakened, and it was more difficult to observe the dust via MODIS’s spectral bands. However, the spatial and temporal distribution of MODIS AOD,
calculated as latitudinal daily averages along the center of
the plume, clearly shows a significant increase of AOD, approximately from 17 February to 1 March (Fig. 7a). The
spatial and temporal distribution of the fine fraction, f , also
calculated as latitudinal daily averages along the center of
the plume, is in good agreement with the spatial and temporal pattern of the AOD (not shown). The suggestion is
that periods of high AOD were characterized by the presence
of a significant coarse fraction, presumably dust. The longitudinal distribution of the AOD, which extended from the
African coast to the South American coast (50◦ W), suggests
the arrival of North African dust as far as the edge of the
Amazon region. Estimation of dust loading (Fig. 7b), which
is contributed to all north African dust sources, proposed
that the extensive dust emission increased the dust loading
over the ocean (between 20◦ N–15◦ S, and 50◦ W–15◦ E) by
more than 100%. The spatial distribution of the AOD from
Atmos. Chem. Phys., 10, 7533–7544, 2010
7540
Y. Ben-Ami et al.: Transport of North African Dust
24/02/2008 23/02/2008 22/02/2008
24/02/2008
21/02/2008 20/02/2008 19/02/2008
39.9
23/02/2008 22/02/2008 21/02/2008
20/02/2008 19/02/2008
90
0.25
-4.93
0
0.4 0
0.4 0
0.4 0
Volume depolarization ratio
0.4 0
0.4 0
0.19 ± 0.03
0.31 ± 0.03
0.22 ± 0.02
0.29 ± 0.03
0.22 ± 0.02
0.21 ± 0.02
4.06
0.19 ± 0.03
13.05
0.28 ± 0.02
0.29 ± 0.03
22.03
0.15 ± 0.03
Latitude (deg)
30.98
Occurrence frequency (Cases)
80
70
0.13
0.26
60
50
0.23
0.20
0.17
40
30
0.32
0.15
20
10
0.4
MB
A D D
0
0 0.25 0.5
0 0.25 0.5
0 0.25 0.5
0 0.25 0.5
0 0.25 0.5
0 0.25 0.5
Volume depolarization ratio
Fig. 6b. Frequency of occurrence of VDR (as in Fig. 5, but for
all days) obtained from CALIOP nighttime profiles. Rare incidences of VDR > 0.5 are not shown. VDR regions dominated by
air molecules (M), anthropogenic (A), biomass-burning (B) and
dust (D) aerosol are marked in red.
a. AOD
0.5
3.4
Elemental analysis (by PIXE) of aerosols collected
at the AMAZE-08 field site in Central Amazonia
The arrival of the dust in the Amazon region was observed at
the AMAZE-08 field site between 22 and 26 February. Elemental analysis by PIXE of the aerosol showed an increase
in the elements Si, Al, Fe, Mn and Ti in the coarse and the
fine fraction, all attributed to mineral dust emitted during 18–
19 February (and possibly even earlier, during 11–16 February) from the Bodélé depression (Fig. 8). The significant increase in the observed crustal elements continued until about
4 March. During this period, the daily averaged concentration of the previously mentioned crustal elements increased
by approximately one order of magnitude (i.e., from 0.076 to
0.79 µg m−3 and from 0.062 to 0.82 µg m−3 in the fine and
coarse modes, respectively). The total mass of the above
crustal elements during this period (between 22 February
and 4 March) was 4.83 µg m−3 . Increases in other elements
Atmos. Chem. Phys., 10, 7533–7544, 2010
b. Dust mass
1.5
2
5/2
5/2
14/2
24/2
17 February to 1 March is composed of minor and major
peaks (marked in Fig. 7a) that are simultaneous with the two
events of dust emissions that occurred during 11–16 and 18–
27 February, including the signature of these emissions in the
Ilorin AERONET measurements (14–16 and 20–22 February). The upper part of Fig. 7a–b (∼ 5–9 February) show
evidence for other period of emission.
1
2
14/2
1
Day
Fig. 6a. Vertical averaged VDR profiles of the aerosol plumes. The
edges of the plumes are defined based on different backscatter patterns (see Sect. 2). Clouds were removed based on their relatively
high VDR and on their spatial distribution. Aerosols located in the
marine boundary layer (the height of the marine boundary layer was
defined based on the location of marine stratocumulus clouds, BenAmi et al., 2009) were excluded from the analysis. Average VDR
(± one standard deviation) for selected regions of pure dust with
VDR between 0.25 and 0.4 are marked in red, and regions of mixed
biomass aerosol with dust (0 < VDR < 0.25) are marked in green.
The blue-marked VDR along the aerosol plumes are those that are
not classified by red or green.
5/3
15/3
50 W
24/2
5/3
30 W
10 W
Longitude (deg)
10 E
15/3
0
10 20 30 40 50
6
Daily dust mass (tonne) ×10
Fig. 7. Daily distribution of AOD and estimation of dust mass. Note
that the area cover by (a) and (b) is not similar: (a) daily AOD distribution over the oceanic region. Each line of the figure is a daily
latitudinal average for the rectangle region between 10◦ N–5◦ S and
50◦ W–10◦ E. The data is based on MODIS AOD maps, in spatial resolution of one degree, as obtained from the Aqua and Terra
satellites. The signature of dust emitted during (1) 11–16 February
and (2) 18–27 February is indicated; (b) daily estimated dust mass
(tonne) over the Atlantic Ocean between 20◦ N–15◦ S and 50◦ W–
15◦ E using Eq. (1–4). Uncertainty in the mass estimate is calculated based on ±30% (dashed lines). Note that in cases when total
AOD > 0.4, the error may be larger.
(not shown), such as S (in the coarse fraction), also occurred,
most likely contributed by marine sources, possibly originating from dimethyl sulfide (DMS) oxidation. Chlorine also in
the coarse fraction indicated that the dusty air plumes passed
over the ocean and mixed with sea salt, as expected. Potassium, in the fine and coarse fraction, may be attributed either to arrival of dust or to a contribution of biomass burning
aerosol, possibly originating from the Southern Sahel region.
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Y. Ben-Ami et al.: Transport of North African Dust
7541
Diameter betwen 2-10 micron
500
AL
Si
Fe
Mn
Ti
0
Diameter < 2 micron
1000
500
AL
Si
Fe
Mn
Ti
30
20
10
0
10-14/2
14-16/2
16-19/2
19-22/2
22-26/2
26-29/2
29/2-4/3
4-8/3
8-12/3
12-21/3
21-30/3
30/3-2/4
2-5/4
5-9/4
9-14/4
14-22/4
22-28/4
28/4-5/5
5-9/5
9-13/5
13-16/5
16-20/5
20-27/5
27/5-2/6
2-13/6
13-19/6
0
Fig. 8. Elemental composition of aerosol collected at the AMAZE08 field site in Central Amazonia during February and June 2008,
(a) fine (diameter < 2.0µm) and (b) coarse fraction (2.0 to 10 µm).
The area in the green rectangle marks the dates for which crustal elements were enhanced. The left Y-axis refers to the concentrations
of Al, Si, and Fe. The right Y-axis refers to the concentrations of
Mn and Ti.
The increase in crustal elements during additional periods,
as seen in Fig. 8 (e.g.: during 5–9 April), suggests that this
is not a singular event and that there are additional cases of
North African dust transport to the Amazon Basin. Analysis of CALIOP profiles along the Brazilian coastline (for
24 February, Fig. 3a) showed that the aerosol plume attained
a maximum height near 3.3 km, while its base merged with
the underlying boundary layer, in agreement with Ansmann
et al. (2009).
3.5
Forward trajectories of air parcels originated from
the Bodélé depression.
Forward trajectories, calculated using the HYSPLIT transport model, starting over the Bodélé depression at the levels of 0, 500, and 1000 m above ground level, propose that
the increase of crustal elements between 22 and 26 February was contributed to dust emitted from the Bodélé (and the
nearby sources) between ∼ 14 and ∼ 15 February (Fig. 9a).
Similarly, the increase of crustal elements between 26 February and 4 March can be attributes to emission of dust from
∼ 18 February and the following days (and partly to emission
from previous days, Fig. 9b). The results of the model proposed that the dust arrived to the heart of the Amazon Basin
(near 60◦ W) in height between 1700 and 5000 m above
ground level, and the transport time from the Bodélé to the
Amazon Basin is between ∼ 10 and ∼ 17 days.
www.atmos-chem-phys.net/10/7533/2010/
b.
20
10
0
a.
30
ng m-3 (for Mn and Ti)
ng m-3 (for Al, Si and Fe)
1000
Fig. 9. Forward trajectories calculated using the HYSPLIT model,
starting from several locations over the region of the Bodélé depression from the period between (a) 14 and 15 and (b) for 18 February 2008. The green, red and blue lines mark the height of the
starting point at 0, 500 and 1000 m above ground level. The region
near Manaus is marked by a white circle.
4
Conclusions
Koren et al. (2006) suggested the Bodélé as source of nutrients for the Amazon Basin based on inductive reasoning without showing evidence for dust deposition. Here we
present a detailed case study that closely follows dust emitted
mainly from the Bodélé depression in Chad and deposited in
the Amazon Basin. The AMAZE-08 measurements in the
central Amazon show episodes of dust, biomass burning and
marine aerosol elements, in agreement with the expected arrival time of the dust-laden air from the Bodélé, as retrieved
by remote sensing measurements. The arrival of dust in the
Amazon Basin is also in line with simultaneous in-situ measurements from AMAZE-08, showing enhancement in ice
nuclei due to the presence of the dust particles (Prenni et al.,
2009).
Nonetheless, despite the dominance of the Bodélé dust
emissions, it is fair to assume contributions from additional dust sources along the transport path. We also assume that not all dust episodes (during winter in the Northern Hemisphere) reach the Amazon forest. For example,
Lidar observations from one month before our case study
(15 January to 14 February 2008), showed that dust/biomassburning plumes that originated from North Africa and were
transported westward, were almost depleted of dust particles when they reached the Amazon region (Ansmann et al.,
2009).
The Bodélé emission rates might be underestimated due
to saturation of the AOD inversion algorithm (at ∼ AOD = 4)
and the use of an AOD-to-mass conversion constant that is
based on a compilation of many in-situ dust measurements
and therefore should reflect average dust events and underestimate strong dust events. Moreover, our data is limited
to the last daily satellite snapshots taken around 13:30 local
time and therefore do not include dust that was emitted during the local afternoon (after the pass of the Aqua satellite
over the region). Based on the spatial and temporal distribution of the VDR and the estimation of dust loading, we show
Atmos. Chem. Phys., 10, 7533–7544, 2010
7542
that the Bodélé’s activity is clearly reflected by changes in
aerosol size distribution and dust loading over the tropical
Atlantic.
Following the two emission periods (between 11–
16 February and between 18–27 February) and based on the
relatively week emission between 11–13 February, the travel
time from the source region to the heart of the Amazon Basin
would be approximately between 8 and 12 days. The HYSPLIT results partly agree with the above suggestion.
Although further studies are needed, our study suggests
that in addition, events with early emission starting time
(around midnight) are likely to be strong dust events and are
more likely to be transported and to reach the Amazon forest
(during the winter season).
Acknowledgements. This research was partly funded by the Minerva foundation, with funding from the Federal German Ministry
for Education and Research, and the German Israeli Science
Foundation (GIF) Contract no. I-899-228.10/2005. Y. R. acknowledges support by the Helen and Martin Kimmel Award for
Innovative Investigation. B. A. Y. acknowledges Jeremy Seltzer for
proofreading.
Edited by: J. Schneider
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